18.5: Enzymes

A catalyst is any substance that increases the rate or speed of a chemical reaction without being changed or consumed in the reaction. Enzymes are biological catalysts, and nearly all of them are proteins. The reaction rates attained by enzymes are truly amazing. In their presence, reactions occur at rates that are a million (106) or more times faster than would be attainable in their absence. What is even more amazing is that enzymes perform this function at body temperature (~37°C) and physiological pH (pH ~7), rather than at the conditions that are typically necessary to increase reaction rates (high temperature or pressure, the use of strong oxidizing or reducing agents or strong acids or bases, or a combination of any of these). In addition, enzymes are highly specific in their action; that is, each enzyme catalyzes only one type of reaction in only one compound or a group of structurally related compounds. The compound or compounds on which an enzyme acts are known as its substrates.

Hundreds of enzymes have been purified and studied in an effort to understand how they work so effectively and with such specificity. The resulting knowledge has been used to design drugs that inhibit or activate particular enzymes. An example is the intensive research to improve the treatment of or find a cure for acquired immunodeficiency syndrome (AIDS). AIDS is caused by the human immunodeficiency virus (HIV). Researchers are studying the enzymes produced by this virus and are developing drugs intended to block the action of those enzymes without interfering with enzymes produced by the human body. Several of these drugs have now been approved for use by AIDS patients.

Table \(\PageIndex{1}\): Classes of Enzymes

Class

Type of Reaction Catalyzed

Examples

oxidoreductases

oxidation-reduction reactions

Dehydrogenases catalyze oxidation-reduction reactions involving hydrogen and reductases catalyze reactions in which a substrate is reduced.

transferases

transfer reactions of groups, such as methyl, amino, and acetyl

Transaminases catalyze the transfer of amino group, and kinases catalyze the transfer of a phosphate group.

hydrolases

hydrolysis reactions

Lipases catalyze the hydrolysis of lipids, and proteases catalyze the hydrolysis of proteins

lyases

reactions in which groups are removed without hydrolysis or addition of groups to a double bond

Decarboxylases catalyze the removal of carboxyl groups.

isomerases

reactions in which a compound is converted to its isomer

Isomerases may catalyze the conversion of an aldose to a ketose, and mutases catalyze reactions in which a functional group is transferred from one atom in a substrate to another.

ligases

reactions in which new bonds are formed between carbon and another atom; energy is required

Synthetases catalyze reactions in which two smaller molecules are linked to form a larger one.

The first enzymes to be discovered were named according to their source or method of discovery. The enzyme pepsin, which aids in the hydrolysis of proteins, is found in the digestive juices of the stomach (Greek pepsis, meaning “digestion”). Papain, another enzyme that hydrolyzes protein (in fact, it is used in meat tenderizers), is isolated from papayas. As more enzymes were discovered, chemists recognized the need for a more systematic and chemically informative identification scheme. In the current numbering and naming scheme, under the oversight of the Nomenclature Commission of the International Union of Biochemistry, enzymes are arranged into six groups according to the general type of reaction they catalyze (Table \(\PageIndex{1}\)), with subgroups and secondary subgroups that specify the reaction more precisely.

Each enzyme is assigned a four-digit number, preceded by the prefix EC—for enzyme classification—that indicates its group, subgroup, and so forth. This is demonstrated in Table \(\PageIndex{2}\) for alcohol dehydrogenase. Each enzyme is also given a name consisting of the root of the name of its substrate or substrates and the -ase suffix. Thus urease is the enzyme that catalyzes the hydrolysis of urea.

Table \(\PageIndex{2}\): Assignment of an Enzyme Classification Number

Alcohol Dehydrogenase: EC 1.1.1.1

The first digit indicates that this enzyme is an oxidoreductase; that is, an enzyme that catalyzes an oxidation-reduction reaction.

The second digit indicates that this oxidoreductase catalyzes a reaction involving a primary or secondary alcohol.

The third digit indicates that either the coenzyme NAD+ or NADP+ is required for this reaction.

The fourth digit indicates that this was the first enzyme isolated, characterized, and named using this system of nomenclature.

The systematic name for this enzyme is alcohol:NAD+oxidoreductase, while the recommended or common name is alcohol dehydrogenase.

Reaction catalyzed:

Summary

An enzyme is a biological catalyst, a substance that increases the rate of a chemical reaction without being changed or consumed in the reaction. A systematic process is used to name and classify enzymes.

Concept Review Exercise

In the small intestine, sucrose is hydrolyzed to form glucose and fructose in a reaction catalyzed by sucrase.

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